1. Field of the Invention
The present invention relates to a laser diode device suitable for a light source for a high-capacity optical disc or the like.
2. Description of the Related Art
In recent years, a nitride compound laser diode device in which a sapphire substrate, a GaN substrate, an SiC substrate or the like is used has been widely used as a light source for a high-capacity optical disc, and a device with a higher output and high reliability has been progressively developed. In such a nitride compound laser diode device, optical confinement in the perpendicular direction (vertical direction) to the active layer is made by using a light guide layer and a cladding layer.
However, even if the light guide layer and the cladding layer are provided, for an AlGaN mixed crystal composing the cladding layer, the film thickness is restricted due to crack generation to the aluminum content. Thus, in some cases, the film thickness of the cladding layer is not able to be grown to a thickness sufficient for light confinement. It has been reported that in this case, part of light generated in the active layer is exuded to the substrate side, and ripple is observed in FFP (Far Field Pattern) (for example, refer to “Applied Physics Letters,” 1999, Vol. 75, No. 19, pp. 2960-2962 (FIG. 5) and “IEEE Journal of Selected Topics in Quantum Electronics,” 2003, September/October, Vol. 9, No. 5, pp. 1252-1259 (FIG. 9)). Further, the light exuded to the substrate side generates mode coupling with light wave-guided through the active layer, which not only lowers light collection characteristics but also affects laser characteristics (for example, refer to “IEEE Journal of Quantum Electronics,” 2007, January, Vol. 43, No. 1, pp. 16-24 (lines 22 to 30 in the right column of p. 20) and “IEEE Journal of Quantum Electronics,” 2000, December, Vol. 36, No. 12, pp. 1454-1461 (lines 2 to 13 in the left column of p. 1460)).